Industrial doors are frequently used to separate different areas of a building which may be operating under different temperature or humidity conditions. In addition, industrial doors can also be employed to provide noise control between two areas of a plant. One common form of industrial door is a rollup door, in which a flexible curtain is coiled on a drum that is mounted above the doorway. Through power operation of the drum, the curtain can be unwound and moved downwardly to a closed position.
Another form of industrial door is a folding door, such as the type described in U.S. Pat. No. 4,770,224. As disclosed in that patent, the door consists of a pair of flexible curtains, the upper end of each curtain being suspended from a series of trolleys that ride on a guide track which is mounted above the doorway. An endless, power operated cable is connected to the various trolleys, and the cable is reeved in a manner such that movement of the cable in one direction will move the curtains to a closed position, while operation of the drive in the opposite direction will move the curtains to a folded, open position.